Research focused on cellular and circuit mechanisms generating the respiratory rhythm and neural activity patterns in the brainstem of rodents. Experimental studies were performed with isolated in situ perfused brainstem-spinal cord and in vitro brainstem slice preparations from neonatal and mature rats. Previously we have identified the brainstem locus (called the pre-Botzinger complex) containing populations of neurons participating in rhythm generation. We have further developed novel methods for real-time structural and functional imaging of these neurons, as well as neurons in rhythm-transmission circuits, utilizing infrared and differential interference contrast (IR-DIC) imaging performed simultaneously with fluorescence imaging of activity patterns of the neurons labeled with calcium-sensitive dyes. This imaging approach has facilitated identification of respiratory circuit neurons for electrophysiological studies of biophysical and synaptic properties as well as molecular studies of neuron channel and receptor expression. With these approaches, we have imaged the activity and analyzed biophysical properties of respiratory neurons in the neonatal rodent pre-Botzinger complex and rhythm transmission circuits in vitro, providing the most direct experimental evidence to date that rhythm generation involves a network of neurons with specialized cellular properties that endow respiratory circuits with multiple mechanisms for producing respiratory oscillations. Methods for multi-photon imaging that will allow three-dimensional reconstruction of this network in the pre-Botzinger complex are currently under development. Studies of neuronal synaptic interactions and cellular membrane biophysical properties in the pre-Botzinger complex, including with advanced electrophysiolgical approaches such as the "dynamic clamp", continue to support our hybrid pacemaker-network model that was formulated from previous work to explain the generation and control of respiratoy rhythm and pattern in the intact mammalian nervous system. These studies have provided additional evidence that neuronal persistent sodium currents and potassium leak conductances represent critical ionic conductance mechanisms for generation and control of respiratory oscillations. Molecular profiling with RT-PCR of messenger RNA expressed in single functionally identified neurons, as well as immunohistochemical studies, show a profile of sodium, potassium, and neurotransmitter receptor-linked channels consistent with an important role of persistent sodium and leak conductances. Electrophysiological studies have also demonstrated that these cellular conductance mechanisms are critically involved in the regulation of the rhythmic breathing patterns by a diverse set of endogenous neurochemicals that modulate these conductances as well as by physiological control signals including carbon dioxide and oxygen. A particular focus of these latter studies was elucidating neuromodulatory control of respiratory circuit activity by neurons of the brainstem serotonergic system, which is postulated to have a critical function in brain state-dependent control of breathing in vivo and is associated with pathophysiological disturbances of breathing such as those underlying sudden infant death syndrome (SIDS). Electrophysiological studies performed with intact preparations of the rodent brainstem-spinal cord in vitro and in situ have established critical functional interactions between raphe and respiratory circuit neurons and determined the essential modulatory actions of raphe serotonergic neurons in both the neonatal and mature mammalian nervous systems. Furthermore, in vivo studies were conducted with transgenic mice that lack raphe serotonergic neurons and have now shown that abnormal sertonergic modulation of respiratory circuit function causes severe instabilities of breathing and high mortality at birth, thus establishing the essential role of sertonergic neurons for stable homeostatic breathing in vivo. New models for the operation of brainstem respiratory circuits that incorporate multiple neuromodulatory control mechanisms have been formulated to explain how specific brainstem circuit components are controlled. We are currently employing pharmaco- and opto-genetic approaches for neuron population-specific manipulation of activity to further investigate how regulation of activity of different populations of network neurons contributes to respiratory pattern generation in different (patho)physiological states.